The present invention relates in general to vapor deposition techniques for coating substrates, and in particular to a new and useful arrangement for uniformly coating surfaces of revolution by vapor deposition in a high vacuum which optimizes various conditions and parameters.
A deposition of substrates by evaporation in a high vacuum is particularly suited for exactly coating spherical or planar surfaces, such as optical lenses, mirrors, and filters, because of the high precision of this process. In the industrial coating of substrates, however, what matters is not only the obtaining of a precise layer thickness, but also low manufacturing costs. For this reason, it is sought to place as many substrates as possible in the vapor beam cone, while making allowances for differences in the layer thickness which occur between both the various locations of the same substrate and identical corresponding locations of different substrates. To reduce these differences to a tolerable degree is a recurrent task.
The simplest way of reducing these differences is, as shown in FIG. 1, to place the substrates in centering holes of a spherically domed plate H, also termed a "dome", so that all the substrate poles P.sub.0, P.sub.1, etc. are equidistantly spaced from the common point of intersection S of the substrate axes A.sub.0, A.sub.1, A.sub.2, etc. The dome is then rotated in the high-vacuum container about a vertical axis A.sub.0 passing through the pole and the center of curvature of the dome.
In this application the term "substrate pole" is used to mean the center of the surface to be coated (a point); and "substrate axis" is used to mean the normal set up at the substrate pole (that is a line which is normal to the plane containing the substrate pole).
If the vapor source O, for example a tantalum plate "dish" filled with magnesium fluoride and heated with electric current, is placed at the point of intersection S of the substrate axes, the substance evaporating in the high vacuum condenses on the substrates as a thin layer having a considerably varying thickness, due to influencing factors, such as the angle of emission .phi., distance r, and angle of incidence .theta., which vary from point to point. On the planar substrate shown by way of example in FIG. 1, in full scale, the layer thickness of an outer substrates at its outer edge, correspond to only about 87%, and that at its inner edge to about 96% of the layer thickness at pole P.sub.0 for the center substrate. This coating is illustrated for substrate wich pole P.sub.1, in FIG. 2a in the form of an isogram by means of lines of equal level, or isolines, with the numbers indicating the proportion of the thickness at the respective locations, to the layer thickness at pole P.sub.0. A substrate in the position P.sub.2 is affected with greater deviation.
The systematic deviations decrease if, with the evaporative source still remaining at the point of intersection of the substrate axes, the angle .alpha..sub.1, formed between the substrate axis A.sub.1 and the axis of rotation A.sub.0 is reduced and the distance R.sub.p from the evaporative source O is increased. This, however, is uneconomical because of the unsatisfactory utilization of the vapor beam cone and too high a manufacturing cost. On the other hand, it is frequently provided to place the evaporative source at such a distance from the axis of rotation A.sub.0, at which the systematic deviations are minimized, for example at a point O' in FIG. 1. What then remains in most instances are undesirable systematic deviations. What is particularly disturbing is the coating asymmetry of substrates which are placed at a larger distance from the evaporative source.
A higher degree of symmetry may be obtained, however, if, in accordance with the well known rules of a two-dimensional parameter optimization and starting from the position P.sub.1, the distance of the substrate pole from the evaporative source is reduced and at the same time, the distance of the point of intersection of the substrate axes with the axis of rotation from the evaporative source is increased, until finally the layer thickness at the substrate pole P'.sub.1 is equal to that at the pole P.sub.0 and becomes the center of a single isoline. By way of example, the isoline obtained on a planar substrate placed in the optimum position P'.sub.1 are shown in FIG. 2b. Since the substrate P'.sub.1, P'.sub.2, etc. are no longer equidistantly spaced from the point of intersection of the substrate axes, supports H', in the shape of stepped spherical zones, are needed which must each be brought into correspondence with the position of the evaporative source and with the basic radius of curvature of the substrates, whereby high manufacturing costs are incurred. Moreover, the unequal coating of substrates rotating in different circular paths is frequently troublesome.
If, on the other hand, circular isolines are desired having their center at the substrate pole and which, strictly speaking, in the arrangement hitherto discussed, can be obtained but with the substrate in its position P.sub.0, a rotation of each of the substrates about its own axis is provided, with the coatings of substrates rotating in addition in a common circular path also remaining always equal to each other. This, however, requires planetary gears or other rotary gearings which are expensive in manufacture and not without problems in operation under the conditions of vapor deposition. Since the vapor beam cone is only unsatisfactorily utilized by substrates involving in a single circular path, the manufacturing costs are very high.
These coatings however, have the particular property of being equal to each other and being representable by circular isolines having their center at the substrate pole. In the following, this property will be termed "uniformity". Even though this uniformity is only seldom desired, it is frequently obtained. If however, "evenness" is sought, that is a uniform layer thickness, this cannot be thoroughly attained with the above described arrangement which may be supplemented by additional circular paths, adjusting devices, masks or further axes of rotation. A distinction is here to be made between "evenness" that is equality in dimension or thickness and "uniformity" that is equality in shape.